Place an oven rack in the center of the oven. Preheat the oven to 375 degrees F. Spray a heavy baking sheet with vegetable oil cooking spray.
Toss together the sweet potatoes, olive oil and salt on the prepared baking sheet. Roast until golden and tender, about 40 minutes.
For the dressing: In a small bowl, whisk together the yogurt, mascarpone cheese, mustard, maple syrup, salt and pepper until smooth.
Pour the dressing over the potatoes. Add the apples, cranberries and pumpkin seeds. Toss until all the ingredients are coated. Refrigerate for at least 1 hour before serving.
Russet potatoes can be substituted in place of the sweet potatoes. To toast the pumpkin seeds, arrange in a single layer on a baking sheet. Bake in a preheated 350 degrees F oven for 6 to 8 minutes until lightly toasted. Cool completely before using.